Bgp Route Selection - H3C S3610-28P Operation Manual

Operation Manual – IPv4 Routing
H3C S3610&S5510 Series Ethernet Switches
No_Advertise: After received, routes with this attribute cannot be advertised to
other BGP peers.
No_Export_Subconfed: After received, routes with this attribute cannot be
advertised out the local AS or other ASs in the local confederation.

5.1.3 BGP Route Selection

I. Route selection rules
BGP uses the following route selection rules:
Discard routes with unreachable NEXT_HOP first
Select the route with the highest Preferred_value
Select the route with the highest LOCAL_PREF
Select the route originated by the local router
Select the route with the shortest AS-PATH
Select IGP, EGP, Incomplete routes in turn
Select the route with the lowest MED value
Select routes learned from EBGP, confederation, IBGP in turn
Select the route with the smallest next hop cost
Select the route with the shortest CLUSTER_LIST
Select the route with the smallest ORIGINATOR_ID
Select the route advertised by the router with the smallest Router ID
Note:
CLUSTER_IDs of route reflectors form a CLUSTER_LIST. If a route reflector
receives a route that contains its own CLUSTER ID in the CLUSTER_LIST, the
router discards the route to avoid routing loops.
If load balancing is configured, the system selects available routes to implement
load balancing.
II. Route selection with BGP load balancing
The next hop of a BGP route may not be a directly connected neighbor. One of the
reasons is next hops in routing information exchanged between IBGPs are not modified.
In this case, a router finds the direct route via IGP route entries to reach the next hop.
The direct route is called the reliable route. The process of finding a reliable route to
reach a next hop is route recursion.
BGP load balancing based on route recursion means that if reliable routes are load
balanced (suppose three next hop addresses), BGP generates the same number of
next hops to forward packets. Note that BGP load balancing based on route recursion
is always enabled by the switch rather than configured using commands.
5-10
Chapter 5 BGP Configuration